The present invention relates to an optical structural element, a method for the production of a layer, a layer itself or a layer system.
Optical structural elements for effecting the transmission of light in the visible spectrum, comprising at least one layer system on a substrate, are sufficiently known, such as for example, from coated glasses. The function of an optical structural element of this type is based on the principle of interference and for that reason at least one transition is provided between materials having a high and a low index of refraction.
The following definitions are used:
Spectrum of visible light:
Spectral range corresponding to a wavelength of 380 nm to 780 nm
Extinction coefficient:
When light of intensity I.sub.0 impinges on a material of thickness d and at the exit side the intensity is I, then EQU I=I.sub.0 exp(-.alpha.(.sup..lambda.)d) (1)
with EQU k(.sup..lambda.)=.alpha.(.sup..lambda.).sup..multidot..lambda. /4.pi.(2)
where .sup..lambda. is the wavelength of the impinging light, k (.sup..lambda.) is the extinction coefficient, and .alpha.(.sup..lambda.) is the absorption coefficient,
Average extinction constant k:
In a considered spectral range between a lower wave-length .sup..lambda. .sub.u and an upper wavelength .sup..lambda. .sub.o applies for the average extinction constant k: ##EQU1## Transmission:
From (1) the transmission T for light of wavelength .sup..lambda. is obtained (neglecting reflection at the surfaces) as: ##EQU2## While consequently the extinction constant k is a material constant, the transmission is a function of thickness d of the material considered.
Average transmission:
For the average transmission T in a spectral band corresponding to a lower wavelength .sup..lambda. .sub.u and an upper .sup..lambda. .sub.o the following applies: ##EQU3##
For optical structural elements which are to be used in the visible spectral range most layers are typically thinner than 100 nm.
As layer materials of layers effective in the visible spectral range are used primarily oxides of group IVa, such as TiO.sub.2, ZrO.sub.2, HfO.sub.2, of group IVb, here in particular SiO.sub.2, of group IIIa, such as Al.sub.2 O.sub.3, Y.sub.2 O.sub.3, Sc.sub.2 O.sub.3, further of group Va , such as Ta.sub.2 O.sub.5, Nb.sub.2 O.sub.3, and oxides or fluorides of some rare earths, such as for example CeO.sub.2, LaF.sub.2 as well as fluorides of groups IIa and lead fluoride as well as silicon nitride. Some of these materials can also be produced in different modifications which are largely determined by production parameters such as "coating temperature", energy of the coat-forming particles, which modifications, in turn, yield different hardnesses.
The hardness of the layers comprising the stated materials, which are customarily produced by vapor deposition, is relatively low, often lower than the Knoop hardness HK.sub.50g of broad glass which is approximately 650.
Materials such as are used in other subfields of coating technology, namely as wear protection coats for tools, are nitrides, carbides and carbonitrides of groups IVa, Va, and Via, further nitrides of Ti, Al or Al.sub.2 O.sub.3, BN and diamond. For this purpose hard layer systems offering protection against wear and tear must have a total thickness,157 100 nm.
While layer systems protecting against wear of stated minimum thickness, comprising the listed layer materials, with the exception of BN and diamond, have a mean extinction constant, averaged in any given spectral band, of at least 10 nm within the visible light spectrum, which is significantly greater than 5.sup..cndot. 10.sup.-2, and layer systems of the stated thickness have an average transmission In the stated spectral band which is significantly less than 10%; BN and diamond layers are presently not usable at commercially justifiable expenditures for optical purposes in tile visible spectral range.